Electronic paper has been proposed for applications in electronic books, the sub-displays of mobile terminal equipment, the display portions of IC cards, and numerous other portable equipment. One display device which is promising for use in electronic paper employs liquid crystal mixtures forming a cholesteric phase (called cholesteric liquid crystals, or chiral nematic liquid crystals; in this Specification, the term “cholesteric liquid crystals” is used). Cholesteric liquid crystals have such excellent features as semi-permanent display maintenance characteristics (memory characteristics), vivid color display characteristics, high contrast characteristics, and high resolution characteristics.
FIG. 1 depicts the cross-sectional configuration of a liquid crystal display device using cholesteric liquid crystals and capable of full-color display. The liquid crystal display device 1 has a structure in which are layered, in sequence from the display face on the side of the user 3, a blue display portion 10, green display portion 11, red display portion 12. In the figure, the upper substrate side is the display face; ambient light 2 is incident from above the substrate on the display face.
The blue display portion 10 has liquid crystals for blue display 10LC sealed between a pair of upper and lower substrates 10A and 10B, and a driving circuit 10P which applies prescribed voltage pulses to the blue liquid crystal layer 10LC. The green display portion 11 has liquid crystals for green display 11LC sealed between a pair of upper and lower substrates 11A and 11B, and a driving circuit 11P which applied prescribed voltage pulses to the green liquid crystal layer 11C. And, the red display portion 12 also has liquid crystals for red display 12LC sealed between a pair of upper and lower substrates 12A and 12B, and a driving circuit 12P which applies prescribed voltage pulses to the red liquid crystal layer 12LC. A light absorption layer 13 is arranged on the rear face of the lower substrate 12B of the red display portion 12.
The cholesteric liquid crystals used in each of the blue, green, and red liquid crystal layers 10LC, 11LC, 12LC are liquid crystal mixtures in which chiral additives (also called chiral materials) are added in relatively large amounts of several tens of weight percent to nematic liquid crystals. When relatively large amounts of chiral materials are intermixed with nematic liquid crystals, a cholesteric phase can be formed in which nematic liquid crystal molecules are strongly twisted in a helical shape. Consequently cholesteric liquid crystals are also called chiral nematic liquid crystals.
Cholesteric liquid crystals have bistable (memory) properties, and by regulating the strength of the electric field applied to the liquid crystals, can assume a planar state (reflecting state), focal conic state (transmitting state), or a state intermediate between these through intermixing thereof. And, once cholesteric liquid crystals have assumed a planar state, a focal conic state, or a state intermediate therebetween, that state is held with stability even after the electric field is no longer applied.
The planar state is for example obtained by applying a prescribed high voltage across the upper and lower substrates to impart a strong electric field to the liquid crystal layer, and after putting the liquid crystals into the homeotropic state, suddenly reducing the electric field to zero. The focal conic state is for example obtained by applying a prescribed voltage, lower than the above high voltage, across the upper and lower substrates to impart an electric field to the liquid crystal layer, and then suddenly reducing the electric field to zero. Or, the focal conic state can also be obtained by gradually increasing the voltage from the planar state. A state intermediate between the planar state and the focal conic state can for example be obtained by applying, across the upper and lower substrates, a voltage lower than the voltage used to obtain the focal conic state, to impart an electric field to the liquid crystal layer, and then suddenly reducing the electric field to zero.
FIG. 2A and FIG. 2B depict the principle of display of liquid crystal display devices using cholesteric liquid crystals. In FIG. 2A and FIG. 2B, an example of a blue display portion is explained. FIG. 2A depicts a state of orientation of cholesteric liquid crystal molecules LC when the liquid crystals for blue display 10LC of the blue display portion 10 are in the planar state. As depicted in FIG. 2A, the liquid crystal molecules LC in the planar state sequentially rotate in the substrate thickness direction to form a helical structure, and the helical axis of this helical structure is substantially perpendicular to the plane of the substrates.
In the planar state, light at a prescribed wavelength according to the helical pitch of the liquid crystal molecules is selectively reflected by the liquid crystal layer. If the average refractive index of the liquid crystal layer is n, and the helical pitch is p, then the wavelength λ of maximum reflection is given by λ=n·p. Hence if the average refractive index n and helical pitch p are determined such that for example λ=480 nm, then the blue liquid crystal layer 10LC of the blue display portion 10 selectively reflect blue light when in the planar state. The average refractive index n can be adjusted by selecting the liquid crystal material and chiral material, and the helical pitch p can be adjusted by adjusting the chiral material content.
FIG. 2B depicts the state of orientation of cholesteric liquid crystal molecules when the blue liquid crystal layer LC of the blue display portion 10 is in the focal conic state. As depicted in FIG. 2B, liquid crystal molecules in the focal conic state rotate successively in substrate in-plane directions, forming a helical structure, and the helical axis of the helical structure is substantially parallel to the substrate plane. In the focal conic state, the reflection wavelength selectivity of the blue liquid crystal layer 10LC is lost, and nearly all incident light 2 is transmitted. And, transmitted light is absorbed by the light absorption layer 13 provided on the rear face of the lower substrate 12B of the red display portion 12, so that a dark color (black) is displayed.
In a state intermediate between the planar state and the focal conic state, the proportion of reflected light to transmitted light can be adjusted according to the state, so that the intensity of reflected light can be varied. Thus when using cholesteric liquid crystals, the amount of reflected light can be controlled through the state of orientation of liquid crystal molecules twisted in a helical shape.
Similarly to the above-described blue liquid crystal layer, when cholesteric liquid crystals which selectively reflect green or red light while in the planar state are sealed into the green liquid crystal layer and red liquid crystal layer respectively, a full-color liquid crystal display device can be realized.
Using cholesteric liquid crystals as described above, by layering liquid crystal display panels which selectively reflect red, green, and blue light, a full-color display device with memory properties is possible, and color display with zero power consumption except when performing screen rewrites is possible.
FIG. 3 depicts reflectivity characteristics versus driving voltage for cholesteric liquid crystals. When a strong electric field (high voltage V1) is applied to liquid crystals, the helical structure of the liquid crystal molecules is completely undone, and all the molecules enter the homeotropic state HT, conforming to the direction of the electric field. When the electric field is suddenly dropped to zero from the homeotropic state HT, the liquid crystal helical axis becomes perpendicular, and the planar state PL is entered. On the other hand, when, from the planar state PL, an electric field (voltage V2) is applied which is sufficiently weak so that the liquid crystal molecule helical structure is not undone, and then the electric field is removed, the focal conic state FC results. And, when an intermediate electric field (voltages V4, V3) is applied and then suddenly removed, an intermediate state, in which the planar state and the focal conic state are intermixed, results.
When liquid crystals are driven using voltage pulses, if the initial state is the planar state PL, then if the pulse voltage is approximately the voltage V2 the focal conic state FC can be induced, and if the pulse voltage is set higher to the voltage V1 the planar state PL can be induced. If the initial state is the focal conic state FC, when the pulse voltage is set to approximately the voltage V2 the focal conic state FC can be induced, and when the pulse voltage is set higher to the voltage V1 the planar state PL can be induced. And, by applying a voltage in a grayscale region A, B from the planar state PL, a grayscale state can be induced.
On the other hand, the driving waveform may be made an alternating current waveform in order to suppress degradation of the liquid crystal material. By using AC driving, image sticking due to the liquid crystal material can be suppressed, and the lifetime of the liquid crystal material can be extended. In general liquid crystal display panels which display video and similar, a frame inversion method which inverts the pulse polarity for each frame, and a line inversion method which inverts the pulse polarity for each scan line, are adopted. In such methods, an AC voltage can be applied to the liquid crystal material, to which positive pulses and negative pulses are applied in alternation over intervals of a plurality of frames.
However, display panels using cholesteric liquid crystals employed in electronic paper perform rewriting of the display image corresponding to image data in one frame or a plurality of frames, and moreover the frequency of image rewriting is extremely low. Hence the above-described frame inversion method and scan line inversion method are not suited to cholesteric liquid crystal display panels.
Hence in a cholesteric liquid crystal display panel used in electronic paper, an inline inversion method, in which the pulse polarity is inverted within each scan line, is applied. In the inline inversion method, one scanning electrode is selected and driven, and positive and negative electric fields are applied to the liquid crystals within a scan interval by applying voltages corresponding to the data from the data line. By means of this method, complete AC pulses are applied to all the pixels even in rewriting the display image for one frame, so that degradation of the liquid crystal material can be suppressed, and the lifetime can be extended.
However, in the inline inversion method, the polarity of driving pulses may be inverted within each scan interval, and the increase in power consumption accompanying polarity inversion is a serious problem. As a method of suppressing this power consumption, FIG. 57 in Patent Reference 1 (JP WO 2005/024774 A1) describes a method of inverting by 180° the phase of AC driving pulses at each scan interval, and reducing by half the frequency of AC driving pulses in a panel.
FIG. 4 depicts pulse control signals which control the polarity of driving pulses described in Patent Reference 1. The pulse control signals FR are control signals applied to a driving circuit; the voltage level of driving pulses output by the driving circuit is controlled according to the pulse control signals FR. Hence by making the pulse control signals FR signals (H,L) in each scan interval as in the case of FR1, both a positive electric field and a negative electric field can be applied to the liquid crystal material. On the other hand, by inverting the phase by 180° at each scan interval as in (H,L), (L,H), (H,L) as in the case of FR2, the electric field can be inverted at each scan interval without inverting the electric field applied to the liquid crystals between scan intervals, so that the frequency of the AC electric field applied to the panel can be halved. In this way, by inverting the phase at each scan interval of the pulse control signals FR, the number of times the liquid crystals are charged and discharged can be reduced, and power consumption by the panel can be suppressed.
However, the inventors have discovered that, in the method of FIG. 4, while the power reduction effect is substantial for images with extensive white portions, such as in the display of text, on the other hand, in the case of images for which there are dramatic changes in density upon each image rewrite, the power reduction effect is smaller. That is, the driving circuit comprises a scanning electrode driving circuit which drives scanning electrodes extending in the horizontal direction of the liquid crystal panel, and a data electrode driving circuit which drives data electrodes extending in the vertical direction, and an electric field corresponding to the difference between a scan pulse and a data pulse is applied to the liquid crystals. Together with this, it was discovered that, if the voltage of unselected scan pulses is set to the intermediate value of the ON and OFF data pulse voltages, the polarity of the electric field applied to liquid crystals at unselected scan electrodes is reversed for data ON and OFF values, and the reduction in power consumption in the liquid crystal panel is different depending on a type of rewrite image.